It is known that fluoroalkylsulfonic acids, which can be represented by the general formula RfSO.sub.3 H (with Rf being a fluoroalkyl group), are used as catalysts in various organic reactions but also as the starting materials in the production of various fluoroalkylsulfonates. For instance, trifluoromethanesulfonic acid (CF.sub.3 SO.sub.3 H) belonging to the RfSO.sub.3 H is generally used as the starting material in the production of lithium trifluoromethanesulfonate (CF.sub.3 SO.sub.3 Li) which is used in an electrolyte for a lithium battery. The production of the lithium trifluoromethanesulfonate for the lithium battery is conducted by providing a high purity trifluoromethanesulfonic acid and subjecting the trifluoromethanesulfonic acid to neutralization reaction with a high purity lithium carbonate. The neutralization reaction in this case can be expressed by the following formula: EQU 2CF.sub.3 SO.sub.3 H+Li.sub.2 CO.sub.3 .fwdarw.2CF.sub.3 SO.sub.3 Li+CO.sub.2 +H.sub.2 O (1)
In this case, it is required for the trifluoromethanesulfonic acid (CF.sub.3 SO.sub.3 H) as the starting material to be as pure as possible such that the content of impurities such as sulfuric acid, free fluorine and the like contained is at a level of the order of a ppm.
The production of the RfSO.sub.3 H on an industrial scale may be conducted, for instance, in accordance with a process disclosed in Japanese Patent Publication No. 4218/1955 (hereinafter referred to as Literature 1) as will be described below.
The process described in Literature 1 comprises the following three steps. That is, in a first step, an alkylsulfonylhalide represented by the general formula RSO.sub.2 X (with R being a non-substitutedalkyl group and X being Cl or F) is provided, and the RSO.sub.2 X compound is subjected to electrolysis in the presence of hydrogen fluoride (HF) to obtain a fluoroalkylsulfonylhalide. This first step can be expressed by the following reaction formula (2), wherein a non-substituted alkylsulfonylfluoride (R SO.sub.2 F) is used as the RSO.sub.2 X and, fluoroalkylsulphonylfluoride is obtained as a reaction product. EQU RSO.sub.2 F+n HF.fwdarw.RfSO.sub.2 F+n/2 H.sub.2 .uparw.+n/2 HF (2)
In a second step, the fluoroalkylsulfonylfluoride (RfSO.sub.2 F) obtained in the first step is subjected to hydrolysis using a KOH solution to obtain a potassium fluoroalkylsulfonate (RfSO.sub.3 K). This second step can be expressed by the following chemical reaction formula (3). EQU RfSO.sub.2 F+2 KOH.fwdarw.RfSO.sub.3 K+KF+H.sub.2 O (3)
In a third step, the potassium fluoroalkylsulfonate (RfSO.sub.3 K) obtained in the second step is mixed with an excessive amount of concentrated sulfuric acid (of 100% in concentration) and if necessary, additionally fuming sulfuric acid, followed by vacuum distillation while heating, to thereby obtain a crude fluoroalkylsulfonic acid (RfSO.sub.3 H) as a final product. This third step can be expressed by the following reaction formula (4). EQU RfSO.sub.3 K+H.sub.2 SO.sub.4 .fwdarw.RfSO.sub.3 H+KHSO.sub.4 ( 4)
The fluoroalkylsulfonic acid (RfSO.sub.3 H) obtained is not pure enough and is contaminated with a distinguishable amount of impurities including sulfuric acid and free fluorine components. Therefore, it is necessary to be refined by subjecting it to rectification. However, it is difficult for the fluoroalkylsulfonic acid to be purified as desired for the following reasons.
In the process described in Literature 1, as above described, since the final step (that is, the third step) is conducted by adding concentrated sulfuric acid and if necessary, additionally fuming sulfuric acid in an excessive amount to the potassium fluoroalkylsulfonate (RfSO.sub.3 K) obtained in the second step to obtain a mixture, and subjecting the mixture to vacuum distillation while heating the mixture, not only a thermal decomposition reaction expressed by the following reaction formula (5) but also side reactions expressed by the following reaction formulas (6) and (7) are unavoidably occurred, to thereby cause a distinguishable amount of impurities including sulfuric acid and free fluorine components, wherein these impurity components are eventually contaminated into the resulting fluoroalkylsulfonic acid (RfSO.sub.3 H). It is extremely difficult to remove such impurities from the fluoroalkylsulfonic acid in the rectification step. Thus, there is a problem in the process of Literature 1 in that it is extremely difficult to obtain a high purity product of fluoroalkylsulfonic acid (RfSO.sub.3 H) substantially free of those impurity components as above described. EQU CF.sub.3 SO.sub.3 H.fwdarw.COF.sub.2 .uparw.+SO.sub.2 .uparw.+HF.uparw.(5) EQU 2 CF.sub.3 SO.sub.3 H+SO.sub.3 .fwdarw.CF.sub.3 SO.sub.2.OCF.sub.3 .uparw.+SO.sub.2 .uparw.+H.sub.2 SO.sub.4 ( 6) EQU 2 CF.sub.3 SO.sub.3 H+SO.sub.3 .fwdarw.(CF.sub.3 SO.sub.2).sub.2 +H.sub.2 SO.sub.4 ( 7)
In order to eliminate the problems in the process of Literature 1, the present inventors previously proposed a process which enables one to refin a crude fluoroalkylsulfonic acid (RfSO.sub.3 H) accompanied by the foregoing impurities such as sulfuric acid and free fluorine components into a refined one (see, Japanese Patent Laid-open No. 85946/1989 (hereinafter referred to as Literature 2). Particularly, the process of Literature 2 comprises the steps of mixing a crude fluorocarbon compound (specifically, a crude floroalkylsulfonic acid) with sulfuric acid and an active silica to obtain a mixture, subjecting the mixture to heat treatment (that is, decomposition treatment with sulfuric acid) at an elevated temperature under pressure condition while stirring the mixture, and subjecting the resultant to vacuum distillation, to thereby obtain a refined fluoroalkylsulfonic acid (RfSO.sub.3 H) having an improved purity with a slight content of free fluorine components (HF). In the process of Literature 2, the free fluorine components contained in the crude fluoroalkylsulfonic acid can be removed because of the use of the active silica, which serves to chemically react with the free fluorine components to afford gaseous SiF.sub.4 capable of being readily removed at the time of the distillation. Thus, the process of Literature 2 is effective in terms of preventing free fluorine components from contaminating into the resulting refined fluoroalkylsulfonic acid. However, there is still a problem for the process of Literature 2 in that although the contamination of free fluorine components into the resulting sulfuric acid can be prevented, a relatively large amount of sulfuric acid components is still liable to contaminate the resulting refined fluoroalkylsulfonic acid even if the vacuum distillation should be conducted by using a packed column or plate column. In addition to this problem, there is also a problem in the process of Literature 2 in that there is a tendency of causing HSO.sub.3 F by way of a chemical reaction expressed by the following reaction formula: H.sub.2 SO.sub.4 +HF.revreaction.HSO.sub.3 F+H.sub.2 O, wherein the HSO.sub.3 F is sometimes remained without being removed upon the distillation, resulting in contaminating into the resulting refined fluoroalkylsulfonic acid.